Maternal depression is more common four years after the birth of a woman’s first child than at any time during the child’s first year, new research shows.

A Murdoch Childrens Research Institute study of 1500 mothers found that 10 per cent of women reported symptoms of depression a year after the birth of their first child – but this increased to 15 per cent four years after the birth.

Women who experienced depression in early pregnancy or in the first year after giving birth were more likely to have symptoms of depression when their child was four years. But 40 per cent of women who were depressed when their child was four had not previously reported symptoms, suggesting their mental health worsened as their children grew older.

Lead author Hannah Woolhouse said the findings contradicted the prevailing view that mothers were most vulnerable to depression in the first couple of months after giving birth. ”This is one of the first large studies to report the prevalence over time of maternal depression in first-time mothers from pregnancy to four years’ postpartum,” she said.

”The findings show the extent of depression affecting first-time mothers, even up to four years after the birth of their child.”

Principal investigator Stephanie Brown said vulnerability to depression in the post-natal period was not entirely due to hormonal and physiological changes, but also linked to major life changes that occurred for new mothers at this time.

She said the higher rate of depression when children were aged four was not explained by the birth of a second or subsequent child.

In fact, women with only one child at the four-year follow-up reported double the rate of depression (23 per cent) compared to those with two or more children (11 per cent).

This was partly explained by higher levels of disadvantage among women with one child at four-year follow-up, the researchers said, but could also be due to a lack of support as children grew older.

Factors that increased the likelihood of depression at four years included being aged under 25 years at the time of first birth; abuse by an intimate partner; having a low income; and experiencing multiple stressful life events such as divorce or housing problems.

Overall, one in three women reported symptoms of depression at some stage in the first four years after the birth of their first child.

Associate Professor Brown said the findings, which were published in the Journal of Obstetrics and Gynaecology, suggested a need to extend monitoring of maternal mental health and offer support well beyond the immediate period after birth.

Parents needing support can contact the Post and Antenatal Depression Association’s helpline on 1300 726 306.